The invention relates to an exhaust gas aftertreatment device for a motor vehicle, comprising an exhaust pipe and a mixing chamber arranged in the exhaust pipe for mixing an exhaust gas flow with a reducing agent for exhaust gas aftertreatment. The reducing agent can be introduced into the mixing chamber by means of a metering device of the exhaust gas aftertreatment device. The mixing chamber has a wall which is on the input side as viewed in a main flow direction of the exhaust gas flow through the exhaust pipe and in which a first inlet for the exhaust gas is formed. The first inlet extends in regions of a lateral surface region of the input-side wall in such a way that exhaust gas entering the mixing chamber through the first inlet can be set in rotary motion about the main flow direction inside the mixing chamber.
Subjecting the exhaust gas flow to angular momentum at the entrance to a channel is described in US 2014/0044603 A1. In the channel, a first component which is designed in the form of a weir having an opening, and a second component which is connected to the first component, cause the exhaust gas flow to move in a spiral-like manner through the channel. In this case, the first component is oriented transversely with respect to the longitudinal axis of the channel.
A disadvantage of this exhaust gas aftertreatment device is considered to be the fact that it can lead to the formation of deposits of components of the reducing agent.
The object of the present invention is therefore to provide an exhaust gas aftertreatment device of the type mentioned at the outset, in which a formation of deposits of components of the reducing agent can be avoided to a particularly great extent.
The motor vehicle exhaust gas aftertreatment device according to the invention comprises an exhaust pipe and a mixing chamber arranged in the exhaust pipe. The mixing chamber is used to mix an exhaust gas stream with a reducing agent for the exhaust gas aftertreatment. The reducing agent can be introduced into the mixing chamber by means of a metering device of the exhaust gas aftertreatment device. The mixing chamber has a wall which is on the input side as viewed in a main flow direction of the exhaust gas flow through the exhaust pipe. A first inlet for the exhaust gas is formed in the input-side wall. The first inlet extends in regions of a lateral surface region of the input-side wall in such a way that exhaust gas entering the mixing chamber through the first inlet can be set in rotary motion about the main flow direction inside the mixing chamber. In this case, the metering device has an outlet device, and a longitudinal axis of the outlet device is inclined counter to the main flow direction of the exhaust gas flow.
As a result of the first inlet extending in regions of the lateral surface region of the input-side wall and thus being oriented transversely to the main flow direction of the exhaust gas flow, a tangential inflow of the exhaust gas into the mixing chamber through the first inlet is achieved during operation of the exhaust gas aftertreatment device. This brings about angular momentum of the exhaust gas flow inside the mixing chamber, that is to say the rotary motion of the exhaust gas about the main flow direction inside the mixing chamber. The main flow direction coincides with an axial direction or a longitudinal direction of the exhaust pipe. Because of this swirling flow of the exhaust gas inside the mixing chamber, or because of the swirling of the exhaust gas entering the mixing chamber, a particularly long mixing distance is achieved even with a short length relative to the diameter of the exhaust pipe in which the mixing chamber is located. Even in the case of a short exhaust pipe, a very thorough mixing of the exhaust gas with the reducing agent can be ensured.
On account of the inclination of the longitudinal axis of the outlet device counter to the main flow direction of the exhaust gas flow being achieved, the reducing agent exiting the outlet device is supplied to the input-side wall of the mixing chamber where, because of the exhaust gas flow, a particularly high heat input into the mixing chamber is present.
This reduces the susceptibility of the mixing chamber to wetting with the reducing agent. The reducing agent may be in particular a urea-water solution, which is available, for example, under the brand name AdBlue®.
Wall films of the reducing agent that may have accumulated on an inner face of the input-side wall of the mixing chamber can also evaporate, or vaporize, particularly effectively. Deposits of components of the reducing agent can thus be avoided to a great extent, both in the region of the outlet device or nozzle and in a region of the mixing chamber that is remote from the nozzle.
Liquid portions of the reducing agent, which lead to wetting of regions of the mixing chamber, accordingly evaporate particularly well.
A particularly low and homogeneous surface load on the walls of the mixing chamber that are supplied with reducing agent can thus be achieved. In other words, when wetting occurs on the walls of the mixing chamber, it involves a load with a particularly small amount of reducing agent per unit area of the walls. This leads to a particularly low cooling of the mixing chamber and to a particularly effective evaporation of the amount of reducing agent introduced into the exhaust gas aftertreatment device. Furthermore, particularly high metering rates can thus be achieved without deposition of chemical secondary products when introducing the reducing agent into the mixing chamber by means of the metering device.
If aqueous urea solution is used as the reducing agent, ammonia is formed in the hot exhaust gas from the urea. A thorough mixing of the released ammonia up until the mixture enters an SCR catalyst makes possible a particularly substantial reaction of the nitrogen oxides in the SCR catalyst with the ammonia. In the SCR catalyst (SCR=selective catalytic reduction), the nitrogen oxide content in the exhaust gas is reduced in a selective catalytic reduction reaction by reacting the nitrogen oxides with ammonia to form nitrogen and water.
A urea preparation can thus be achieved in a flood with the catalysts or with the catalyst or substrate support without deposits forming or with a particularly substantial reduction in deposit formation, even in the case of particularly high metering rates and low temperature.
This also applies in the case of a short length relative to the diameter, which is unfavorable per se. The arrangement of the mixing chamber is suitable for use in series directly downstream of an oxidation catalyst and/or particulate filter or directly upstream of an SCR catalyst or upstream of a particulate filter provided with an SCR-active coating.
The longitudinal axis of the outlet device, or spray axis, can be arranged so as to be inclined at from approximately 15° to approximately 30°, in particular with respect to a cross-sectional plane of the exhaust pipe in which the mixing chamber is located. By virtue of such an inclination angle of the outlet device or nozzle with respect to a vertical axis of the mixing chamber, the reducing agent droplets can be injected against the input-side wall, relatively close to the outlet device or injection nozzle, where the droplets can evaporate particularly quickly.
Preferably, it is possible to supply the reducing agent by means of the outlet device to an inner face of a curved transition region between an end face region and the lateral surface region of the input-side wall. By virtue of such a rounded, i.e., corner- and edge-free, design of the input-side wall, droplets of the reducing agent remaining suspended on corners or edges and accumulating as a liquid wall film can be particularly substantially prevented. This also brings about a particularly low tendency for deposits to form. A baffle surface can be provided in a particularly simple manner on the input-side wall, which is designed in particular in the manner of a round shell, when the input-side wall of the mixing chamber is formed by deep-drawing of a corresponding metal component.
Preferably, it is possible to supply exhaust gas to an outer face of the input-side wall in the transition region. This exhaust gas, which subsequently passes via the first inlet into the mixing chamber, ensures particularly good heating or keeps the mixing chamber warm in the region in which the reducing agent is supplied to the input-side wall during operation of the exhaust gas aftertreatment device. This rear-face convection of exhaust gas in the region of the mixing chamber ensures a particularly large heat input into the mixing chamber where the reducing agent is supplied.
As a result of preferably at least the end face of the mixing chamber and at least the predominant region of the lateral surface of the mixing chamber being formed by the input-side wall, perturbations or edges can be avoided in a particularly large partial region of the mixing chamber. This, too, is conducive to preventing the formation of deposits.
It has been demonstrated to be further advantageous for a length of the first inlet measured in the main flow direction of the exhaust gas flow to be greater than a width of the first inlet measured perpendicularly to the main flow direction of the exhaust gas flow.
Such an elongate geometry of the first inlet allows the exhaust gas to flow into the mixing chamber at a favorable flow rate, and consequently to generate the angular momentum transversely to the main direction or main flow direction of the exhaust gas in the region of the mixing chamber.
Additionally or alternatively, the first inlet and the outlet device are arranged with an angular offset from each other of from approximately 40° to approximately 60° in a circumferential direction of the exhaust pipe. It can thus be ensured in a particularly simple manner that there is a swirling flow inside the mixing chamber in the region of the outlet device, that is to say the rotary motion of the exhaust gas inside the mixing chamber.
The first inlet is preferably arranged in this case upstream of the outlet device as viewed in a flow direction of the exhaust gas set in rotary motion.
For example, the first inlet may be arranged approximately 50° upstream of the outlet device or injection nozzle. The exhaust gas flow flowing through the exhaust pipe in the main flow direction can thus enter the mixing chamber in a particularly simple manner via the first inlet. Nevertheless, during operation of the exhaust gas aftertreatment device in the region of the outlet device, the exhaust gas is already set in rotary motion.
It is furthermore advantageous for the mixing chamber to have a second inlet in the region of the outlet device, via which inlet exhaust gas can be introduced into the mixing chamber. The second inlet is preferably designed as a substantially circular opening through which the longitudinal axis of the outlet device passes. A particularly unhindered entry of the exhaust gas via the second inlet into the mixing chamber is achievable if the opening is formed continuously, i.e., none of the edges of the opening has interconnecting bridges or the like. By introducing exhaust gas via an opening of this kind during operation of the exhaust gas aftertreatment device, it is possible to achieve a concentric envelope of the reducing agent or spray cone which can be introduced into the mixing chamber via the outlet device.
This is also particularly advantageous with regard to preventing the formation of deposits of the reducing agent.
The mixing chamber preferably has at least one third inlet arranged downstream of the outlet device as viewed in a flow direction of the exhaust gas set in rotary motion, via which inlet exhaust gas can be introduced into the mixing chamber. By means of one or more such third inlets, the reducing agent or the spray cone is particularly substantially shielded from the walls of the mixing chamber and distributed over a large area. As a result, deposition formation can be avoided.
The terms “second inlet” and “third inlet” are not to be understood as measure words in the present case; rather, they make it easier to distinguish the inlets from one another. In this respect, the second inlet and/or the third inlet may also be referred to as a “further inlet.” This is true, for example, when only the first inlet and the second inlet or only the first inlet and at least one third inlet are provided.
Preferably, a length of the at least one third inlet measured in the main flow direction of the exhaust gas flow is greater than a width of the at least one third inlet measured perpendicularly to the main flow direction of the exhaust gas flow. Due to this elongate shape of the third inlet, the exhaust gas can enter the mixing chamber in a favorable manner in terms of flow.
Additionally or alternatively, the third inlet and the outlet device are arranged with an angular offset from each other of from approximately 20° to approximately 40° in a circumferential direction of the exhaust pipe. For example, the third inlet may be arranged approximately 30° downstream of the outlet device or nozzle and extend substantially in the longitudinal direction of the exhaust pipe, i.e., in the main flow direction of the exhaust gas flow. The at least one third inlet thus also helps to prevent the formation of deposits.
A sum of the inlet cross sections through which gas can flow preferably corresponds to from approximately 15% to approximately 50% of an exhaust pipe cross section through which gas can flow. This makes it possible for the exhaust gas to enter the mixing chamber in a particularly unhindered manner and so as to lead to a thorough mixing of the reducing agent with the exhaust gas flow.
An outlet for the exhaust gas is preferably provided in an output-side wall of the mixing chamber. An outlet of this kind facilitates the formation of vortices in the exhaust pipe and therefore the thorough mixing of the reducing agent with the exhaust gas flow. The arrangement, shape and orientation of the outlet opening facilitate the swirling flow in the mixing chamber.
Furthermore, downstream of the output-side wall of the mixing chamber, a wall element may be arranged in the exhaust pipe, which element has a plurality of passage openings and which abuts the circumference of an inner face of the exhaust pipe. A wall element of this kind, designed in the manner of a perforated plate, is also conducive to the intensive mixing of the reducing agent with the exhaust gas.
For such thorough mixing, it is also beneficial for a gap to be formed between the mixing chamber and the wall element.
The features and feature combinations mentioned above in the description as well as the features and feature combinations mentioned below in the description of the drawings and/or shown alone in the drawings can be used not only in the respectively specified combinations but also in other combinations or in isolation, without departing from the scope of the invention. The invention may therefore also be considered to comprise and disclose embodiments which are not explicitly shown or explained in the drawings but which emerge and can be produced by separate feature combinations from the embodiments explained. Embodiments and feature combinations which do not have all the features of an originally formulated independent claim may therefore also be considered to be disclosed. Moreover, embodiments and feature combinations which go beyond the feature combinations outlined in the dependency references of the claims or deviate therefrom are considered to be disclosed, in particular by the embodiments outlined above.
Further advantages, features and details of the invention can be found in the claims, the following description of preferred embodiments and with reference to the drawings.